This invention relates generally to methods and apparatus for cooling lamination stacks, and, more particularly, to apparatus and methods for cooling electric motors for use in electric and hybrid vehicles.
Present day electric and hybrid vehicles typically use inverter-driven, radial gap motors that are either AC induction or DC brushless type motors. Both of these motor types are similar in that an interior rotor rotates within an exterior stator. The stator can be made of a lamination stack (i.e., a stack of planar laminations) supporting windings in a conventional arrangement. In forming the lamination stack, the laminations a thermally activated bonding agent is applied to the laminations, and then the laminations are compressed together and heated.
During operation of the motors, heat primarily flows radially outward, with heat produced within the rotor being transferred through an air gap to the stator. Heat from the stator is in turn transferred to an outer housing that is typically composed of aluminum. In some applications the housing is air-cooled. For other air-cooled alternatives, it is known to pass air through passageways formed radially or axially between laminations. In other applications, the housing includes channels through which a liquid coolant is directed. This latter approach, using a “liquid-cooled housing,” is generally favored over air-cooled schemes because it provides improved heat transfer while allowing reduced overall dimensions.
Liquid-cooled housings typically must be a complex “investment casting.” The housing inner diameter (“ID”) and stator outer diameter (“OD”) typically must each be turned or ground to precision dimensions so that an accurate interference fit can be achieved between the two to provide for efficient heat transfer between the stator and the housing. Thermal grease is often added to the stator at its OD prior to installation into the housing to provide for optimal heat transfer between the stator and the housing. To facilitate installation of the stator into the housing, it is often necessary to cool the stator and/or heat the housing prior to assembly. Each of these aspects adds to the expense of manufacturing a liquid cooled housing. A liquid-cooled housing will typically have small gaps between the stator and housing, causing increased thermal impedance between them, thereby reducing the continuous power rating of the motor. Furthermore, the housing add size and weight to the stator and rotor.
In an alternative cooling scheme, it is known to submerge a lamination stack in a liquid coolant bath, and to provide passageways through the laminations for coolant to convectively flow.
It is desirable for a motor to be cooled, preferably with an efficiency as great as or better than by a liquid cooled housing, with a minimum of additional size and weight (i.e., mass) over that required for the stator and rotor. Further, it is desirable to minimize the total cost of manufacturing such a motor cooling system. Various embodiments of the present invention can meet some or all of these needs, and provide further, related advantages.